Low temperature firing multilayer ceramic circuit boards are known that are suitable for use with low melt temperature conductive metals, such as silver, gold and copper. These metals can be screen printed onto green tapes to form a desired circuitry. The multilayer circuit boards have a low thermal coefficient of expansion (TCE) and they may be formulated to be compatible with the TCE of both silicon and gallium arsenide devices. These ceramic circuit boards are made from glasses that can be fired at temperatures of less than 1000.degree. C. They are made by admixing finely divided selected glass particles or powders and optional inorganic fillers with organic materials, such as resin, solvents, dispersant and the like; the resultant slurry is cast as a thin tape, called green tape. A circuit pattern may be screen printed onto the green tape using a conductive ink formulation comprising a conductive metal powder, an organic vehicle and a powdered glass, generally the same as, or a similar glass to, that used to make the green tape.
When a plurality of green tapes are stacked together, via holes are formed in the tapes, which vias are then filled with a conductive via fill ink made with a conductive powder, an organic vehicle and a suitable glass, to provide electrical contact between the circuits on adjoining green tape layers. When all of the desired green tapes have been patterned and the vias are filled, the green tapes are aligned and then laminated under heat and pressure, and fired to remove the organic materials and densify the glass.
More recently, the multilayer ceramic circuit boards have been adhered to a metal support substrate, which increases the strength of the multilayer board. When a suitable bonding glass is used to adhere the green tapes to the support substrate, an additional advantage is obtained because the bonding glass reduces the shrinkage in the x and y dimensions of the green tapes during firing. Thus most of the shrinkage occurs only in the z, or thickness, dimension and the printed circuits can be formed to tighter tolerances. The glasses used in the green tapes however, must have a TCE matched to that of the metal support to prevent delamination or cracking of the fired glass. Mixtures of crystallizable and non-crystallizable glasses can be used for example, and inorganic fillers can also be used to match the TCE of the fired green tape ceramic to that of the metal support.
Green tape glasses suitable for use in the present invention are mixtures of crystallizable glass compositions mixed with non-crystallizing glasses, which can further include oxide fillers. These glasses are particularly useful for metal supported printed ceramic circuit boards. The crystallizing glasses are chosen from ZnO, MgO, B.sub.2 O.sub.3 and silica systems, and preferably are made from 20-55% by weight of ZnO, from 10-30% by weight of MgO, from 10-35% by weight of boron oxide and 10-40% by weight of silica. These glasses can also include up to 10% by weight of alumina and up to 3% by weight of a coloring agent, such as cobalt oxide. These crystallizing glasses are mixed with non-crystallizing glasses or vitreous glasses chosen from lead-based glass compositions including 30-80% by weight of lead oxide, from 15-50% by weight of silica, up to 10% by weight of alumina, up to 15% by weight of boron oxide and up to 20% by weight of zinc oxide. In addition, one or more oxide fillers such as alumina, cordierite, quartz, forsterite and willemite can also be added to control the shrinkage and modify the TCE of the green tape. These glass mixtures are used to form green tapes that can be aligned and laminated together and adhered to the support board. These glasses and mixtures and their green tapes are further described in U.S. Pat. No. 5,725,808 to Torney et al which is herein incorporated by reference.
Particularly useful metal support boards are made of a ferro/nickel/cobalt/manganese alloy, clad with a thin copper layer, available under the trademark KOVAR.RTM. of Carpenter Technology.
Passive components such as resistors and capacitors can also be embedded in a green tape stack. Suitable resistor or capacitor inks can be screen printed onto green tapes to obtain tight tolerances and high precision placement of the passive components.
However, when screen printed inductors were tried using the above techniques, they had low Q values of less than 50, and inductor values of less than several hundred nanohenries (nH).
High value discrete inductors can be formed by surrounding the inductor with a high permeability ferrite. However, this cannot be transferred directly to the fabrication of multilayer embedded green tape inductors because the firing temperature of such ferrites is much higher than the firing temperature of the present green tape glasses. The addition of a flux or sintering aid has been suggested to lower the sintering temperature of ferrites, but these generally have the effect of lowering the permeability of the ferrite as well. Further, ferrites must be compatible with the glasses and other materials used to make the green tapes, and must have a TCE fairly close to that of the ceramics used to make the green tapes. Such constraints are not trivial, but a search has continued for high permeability ferrites that can be used in the present multilayer metal supported ceramic circuit board system.